MSefi.com read only forum (Jan.15,2018)

hi
I have assembled the knock detection circuit as it is in

it's working quite well now. the led on the circuit is always grounded and it gets +ve when it is supposed to give light.
all is good.

the thing is , im trying to connect this led to the original check engine light in the car original panel ..... but the check engine light on the panel workes the other way ... it has +ve all the time and when it's suposed to give light it gets a ground ..... and i only have acess to the ground wire of the check engine light.
where can I connect the ground of the check engine light in the above circuit so that it only lights when the led on the circuit turns on ?

thanks .... and sorry bout my electronic skills

anyone ?

The circuit drives the LED with the 555 output. I thinks that unless you convert your dash light to an LED, a regular light bulb may draw too much current, plus it'a probably rated at 12v instead of the ~5v the circuit produces to drive the LED.

Also, an LED is a diode, which illuminates when forward biased. thus blocking current in the reverse direction. Light bulbs conduct in either direction, so if you installed into the circuit it may just apply 12v to the 555 and the output switch which would probably burn them up.

That's not saying you couldn't make a simple transistor circuit similar to the output stage transistor, as a buffer to switch the 12V bulb on and off with 12V to ground.

Just take another 1K and 2n2222 transistor and put them on pin3 like the output stage but without the 470R. Your bulb ground wire would connect to the transistor collector where it says output. If it wasn't a 5 volt circuit output you could have just connected the bulb right to the output. You'd fry the processor if you did that...

i will try that ... thanks mate